Bacteria are the most numerous of the culturable soil microorganisms. Their numbers are often as high as 1 x 108 per gram of soil. Therefore, a handful of soil may contain more living organisms than there are people on the face of the earth. Bacteria have a wide variety of shapes, sizes, and functions. Some require free-molecular oxygen, some can live without oxygen, and some (facultative anaerobic) can live with or without oxygen. Their sizes usually range from about 0.2 to 3 μm. Soil bacteria are active in many soil nutritive cycles, especially the carbon and nitrogen cycles. The video shows free-living Azotobacter ("azoto" means nitrogen in Russian) growing on a soil surface and bacteria that fix nitrogen symbiotically (in a mutually beneficial relationship) with higher plants. These organisms use nitrogen gas from the air for growth and reproduction. A nodule (round structure that houses the Bradyrhizobium) is shown growing on the root system of a soybean plant. If cutting into the nodule reveals a red color, active nitrogen fixation is occurring. The red color is due to leghemoglobin, a compound produced by the plant that protects the enzyme nitrogenase responsible for fixation from oxygen. There are many nitrogen-fixing legumes. Most beans, such as kidney, green, or lima beans, fix nitrogen from the atmosphere. Garden peas and forage crops such as alfalfa, clover, and trefoil, all fix nitrogen. Some shrubs also fix nitrogen such as alder that grows commonly in the northwest USA and in Alaska. The endophyte, or organism inside the nodule, in this case, is an actinomycete of the genus Frankia.
Figure 1: Soil Bacteria (video with audio)
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